Method of producing epoxy resin film

ABSTRACT

An epoxy resin film which is so strong as to have a tensile strength of not less than 10 MPa and an elongation of not less than 5% is formed by using a substantially linear high molecular weight epoxy resin which is synthesized by polymerizing a difunctional epoxy resin having two epoxy groups per molecule and a dihydric phenol in an aprotic polar solvent, in the presence of a solvent, at a reaction temperature ranging from 80° to 130° C., the difunctional epoxy resin and the dihydric phenol being used in quantities which provide a ratio of phenolic hydroxyl groups to epoxy groups of from 1:0.9 to 1:1.1 and a proportion of the total of them of at most 50% by weight based on the total of them and the aprotic polar solvent.

This application is a divisional application of application Ser. No.757,587, filed Sep. 11, 1991, now U.S. Pat. No. 5,225,268.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to an epoxy resin film and a method ofproducing the epoxy resin film which is excellent in propertiesincluding mechanical properties such as strength and elongation andadhesion properties.

(b) Description of the Related Art

The method of producing high molecular weight epoxy resins by employingrelatively low molecular weight difunctional epoxy resins and dihydricphenols as polymerization materials is generally called two-stagemethod, and the first literature on this method is the specification ofU.S. Pat. No. 2,615,008 which corresponds to the published specificationof Japanese Patent Application filed by the same applicant as theassignee of the U.S. Patent and published under publication No. 28-4494.In these literatures is disclosed a method of producing a highermolecular weight epoxy resin having an epoxy equivalent weight of 5,600by carrying out a reaction at 150° to 200° C. by using sodium hydroxideas a copolymerizing catalyst in the absence of solvents. The averagemolecular weight of the obtained epoxy resin is presumed to be about11,000. In these literatures, however, there is no example wheresolvents are used.

An example of the literatures in which the use of solvents is disclosedis the specification of U.S. Pat. No. 3,306,872. Particular examples ofthe literatures in which the use of solvents in working examples isdisclosed include Japanese Patent Application Kokai Koho (Laid-open) No.54-52200, Japanese Patent Application Kokai Koho (Laid-open) No.60-118757, Japanese Patent Application Kokai Koho (Laid-open) No.60-118757, Japanese Patent Application Kokai Koho (Laid-open) No.60-144323 and Japanese Patent Application Kokai Koho (Laid-open) No.60-114324. The solvents used in these literatures include methyl ethylketone, methyl isobutyl ketone, cyclohexanone, ethylene glycol monoethylether and ethylene glycol monomethyl ether. These solvents areclassified into ketone solvents and ether (cellosolve) solvents.

In the specification of U.S. Pat. No. 3,306,872, either methyl ethylketone or ethylene glycol monomethyl ether is used as a solvent, and theconcentration of the solids content is 20 to 60% by weight. Thecatalysts used therein are hydroxides and phenolates of alkyl metals andbenzyltrimethylammonium. The polymerization reaction is continued at atemperature of 75° to 150° C. until the weight average molecular weightsof the formed high molecular weight epoxy resins increase to at least40,000 or more. The average molecular weights of the obtained highmolecular weight epoxy resins are measured by the viscosity method to be50,000 to 1,000,000. However, it is known that, in the viscosity method,the calculated average molecular weights are very dependent on theparameters established for the calculation, and, therefore, the averagemolecular weights of the high molecular weight epoxy resins produced inthe U.S. Pat. No. 3,306,872 are not entirely accurate.

Another working example wherein a high molecular weight epoxy resin issupposed to be obtained by carrying out the polymerization in a solventis disclosed in Japanese Patent Application Kokai Koho (Laid-open) No.54-52200, in which it is disclosed that a high molecular weight epoxyresin having an average molecular weight of 45,500 is obtained by usingethylene glycol monoethyl ether as a solvent. Further, it is disclosedin Japanese Patent Application Kokai Koho (Laid-open) No. 60-118757 thathigh molecular weight epoxy resins having average molecular weights ofat most 31,000 are obtained by using methyl isobutyl ketone,cyclohexanone or ethylene glycol monoethyl ether as a solvent. InJapanese Patent Application Kokai Koho (Laid-open) No. 60-144323disclosed is the production of a high molecular weight epoxy resinhaving an average molecular weight of 53,200 by the use of methyl ethylketone as a solvent, and in Japanese Patent Application Kokai Koho(Laid-open) No. 60-144324 disclosed is the production of a highmolecular weight epoxy resin having an average molecular weight of66,000 by the use of methyl ethyl ketone as a solvent. In every one ofthese four literatures, the average molecular weights are measured bygel permeation chromatography, but the measuring conditions and thecalculation methods are not disclosed. The molecular weights measured bygel permeation chromatography vary largely depending on the measuringconditions including the kinds of the fillers used and the kinds of theeluents used and the calculating methods so that it is difficult toobtain accurate average molecular weights, and, therefore, thedetermined values of the average molecular weights of the high molecularweight epoxy resins produced in these literatures are not entirelyaccurate.

These high molecular weight epoxy resins which have been knownheretofore are not linear high molecular weight epoxy resins but arebranched high molecular weight epoxy resins, which cannot be formed intofilms of 100 μm or less thickness having sufficient strength.

Further, none of the literatures described above disclose that theobtained high molecular weight epoxy resins are able to be formed intofilm, nor give examples of such epoxy resins. Also, since the obtainedepoxy resins are soluble in the solvents other than amide solvents, itis apparent that the methods disclosed in these literatures could notprovide so called ultra high molecular weight epoxy resins which arelinearly polymerized so highly as to have the capability of being formedinto films having sufficient strength.

A method of producing epoxy resin sheets by using linear high molecularweight epoxy resins is disclosed in Japanese Patent Application KokaiKoho (Laid-open) No. 51-87560. In the method, sheets of 0.3 to 0.5 mmthickness are produced from a mixture prepared by heat-melting linearhigh molecular weight epoxy resins and low molecular weight epoxy resinsand mixing them with salts of organic carboxylic acids. The sheetsobtained by the method have a strength of about 10 MPa and an elongationof 350 to 870%. There is disclosed that the linear high molecular weightepoxy resins used have a molecular weight of 30,000 to 250,000, butthere is no description concerning the method of measuring the molecularweights, so that it is impossible to compare the molecular weights withthe molecular weight of the linear high molecular weight epoxy resinused in the present invention. From working examples which show thecapability of the high molecular weight epoxy resins to be heat-meltedat 190° C., it is presumable that these epoxy resins have astyrene-conversion weight average molecular weight of less than 50,000as determined by gel permeation chromatography. As the results ofmeasurements carried out by the present inventors with a differentialscanning thermometer, it was proved that linear high molecular weightepoxy resins produced from bisphenol A epoxy resin and bisphenol A havemelting points or softening points of 300° C. or higher when theirstyrene-conversion weight average molecular weights are 50,000 or more,and melting points or softening points of 350° C. or higher when theirstyrene-conversion weight average molecular weights are 100,000 or more.That is, the high molecular weight epoxy resins disclosed in JapanesePatent Application Kokai Koho (Laid-open) No. 51-87560 are presumed tohave considerably lower molecular weights or are much more branched, ascompared with the high molecular weight epoxy resins used in the presentinvention.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an epoxy resin filmwhich is thin in thickness and has a high strength by using a linearlypolymerized high molecular weight epoxy resin which has not beenobtainable.

That is, the present invention provides an epoxy resin film which has athickness of not more than 100 μm, a tensile strength of not less than10 MPa and an elongation of not less than 5% and is made of a highmolecular weight epoxy resin being insoluble in methyl ethyl ketone andhaving a substantially linear structure.

The present invention further provides a suitable method of producingthe epoxy resin film of the present invention, which comprises

applying a solution of a high molecular weight epoxy resin dissolved inan aprotic polar solvent at a concentration of at most 50% by weight toa plate having a release surface to coat the release surface with thesolution of the high molecular weight epoxy resin, the high molecularweight epoxy resin being insoluble in methyl ethyl ketone, having asubstantially linear structure, and being prepared by polymerizing adifunctional epoxy resin having two epoxy groups per molecule and adihydric phenol in an aprotic polar solvent, at a reaction temperatureranging from 80° to 130° C., the difunctional epoxy resin and thedihydric phenol being used in quantities which provide a ratio ofphenolic hydroxyl groups to epoxy groups of from 1:0.9 to 1:1.1 and aproportion of the total of the difunctional epoxy resin and the dihydricphenol of at most 50% by weight based on the total of the difunctionalepoxy resin, the dihydric phenol and the aprotic polar solvent and

removing the aprotic polar solvent from the solution of the highmolecular weight epoxy resin applied to the release surface of theplate, to form an epoxy resin film having a thickness of not more than100 μm.

THE PREFERRED EMBODIMENTS OF THE INVENTION

The difunctional epoxy resin to be used in the method of the presentinvention may be any compound as far as it contains two epoxy groups permolecule, and some examples include bisphenol A epoxy resin, bisphenol Fepoxy resin, bisphenol S epoxy resin, alicyclic epoxy resins, aliphaticlinear epoxy resins, diglycidyl ethers of dihydric phenols diglycidylethers of dihydric alcohols, halides thereof and hydrides thereof. Themolecular weights of these compounds are not limited. These compoundsmay be used individually or as a mixture of two or more of them. Thepreferred is bisphenol A epoxy resin. Some ingredients other than thedifunctional epoxy resin may be contained as impurities so far as theydo not affect the operation and accomplishment of the present invention.

The dihydric phenol to be used in the method of the present inventionmay be any compound as far as it contains two phenolic hydroxyl groupsper molecule, and some examples include monocyclic dihydric phenols,such as hydroquinone, resorcinol and catechol, polycyclic dihydricphenols, such as bisphenol A and bisphenol F, halides thereof and alkylsubstituted compounds thereof. The molecular weights of these compoundsare not limited. These compounds may be used individually or as amixture of two or more of them. The preferred examples includehydroquinone, resorcinol, bisphenol A and tetrabromobisphenol A, and theparticularly preferred examples are hydroquinone, resorcinol andbisphenol A. Some ingredients other than the dihydric phenol may becontained as impurities so far as they do not affect the operation andaccomplishment of the present invention.

The catalyst to be used in the method of present invention may be anycompound as far as it has the catalytic function of accelerating theetherification between the epoxy groups and the phenolic hydroxylgroups. Some examples of the catalyst include alkali metal compounds,alkaline-earth metal compounds, imidazoles, organic phosphorus compoundssuch as alkyl phosphines, secondary amines, tertiary amines andquarternary ammonium salts. The preferred are alkali metal compounds,such as hydroxides, halides, organic acid salts, alcoholates,phenolates, hydrides, borohydrides and amides of sodium, lithium andpotassium. The particularly preferred examples include sodium hydroxide,lithium hydroxide and sodium methoxide. These catalysts may be usedindividually or as a mixture of two or more of them.

The aprotic polar solvent to be used in the method of the presentinvention may be any aprotic polar solvent as far as it dissolves thereaction materials, namely the difunctional epoxy resin and the dihydricphenol, and the product high molecular weight epoxy resin. The preferredexamples include amide solvents, such as formamide, N-methylformamide,N,N-dimethylformamide, acetamide, N-methylacetamide,N,N-dimethylacetamide, N,N,N',N'-tetramethylurea, 2-pyrrolidone,N-methylpyrrolidone and carbamates. These solvents may be usedindividually or as a mixture of two or more of them. Further, theseaprotic polar solvents may be used together with other solvents, such asketone solvents and ether solvents, so far as the operation andaccomplishment of the present invention are not affected.

As to the polymerization conditions in the method of the presentinvention, the difunctional epoxy resin and the dihydric phenol are usedin quantities which provide a ratio of phenolic hydroxyl groups to epoxygroups of from 1:0.9 to 1:1.1. When the ratio of phenolic hydroxylgroups to epoxy groups is less than 0.9, the increase of the molecularweight by linear polymerization does not proceed, but crosslinking iscaused by side reactions, resulting in a product insoluble in theaprotic polar solvent. When the ratio of phenolic hydroxyl groups toepoxy groups is more than 1.1, the increase of the molecular weight doesnot proceed sufficiently.

Although the quantity of the catalyst is not particularly limited, thecatalyst, generally, is used in a quantity of 0.0001 to 0.2 mol per 1mol of the difunctional epoxy resin. When the quantity of the catalystis less than the range, the increase of the molecular weight may beslowed down severely, and when it is larger than the range, sidereactions may increase, thereby preventing the increase of the molecularweight by linear polymerization.

The preferred range of the reaction temperature is 80° to 130° C. Whenthe reaction temperature is lower than 80° C., the increase of themolecular weight may be slowed down severely, and when it is higher than130° C., side reactions may increase, thereby preventing the increase ofthe molecular weight by linear polymerization.

The solids concentration during the polymerization, namely theproportion of the total of the difunctional epoxy resin and the dihydricphenol based on the total of the difunctional epoxy resin, the dihydricphenol and the aprotic polar solvent, is at most 50% by weight. Thepreferred concentration is less than 30% by weight. The higher thesolids concentration is, the more the side reactions occur to make theincrease of the molecular weight by linear polymerization difficult.When the polymerization is carried out with a relatively high solidsconcentration, lowering the reaction temperature and decreasing thequantity of the catalyst used make it possible to obtain linear ultrahigh molecular weight epoxy resins.

The high molecular weight epoxy resin obtained by using theabove-described difunctional epoxy resin, dihydric phenol, catalyst andaprotic polar solvent according to the present invention is insoluble inmethyl ethyl ketone, and it generally has a styrene-conversion weightaverage molecular weight of more than about 20,000 as measured by gelpermeation chromatography. For example, a highly branched high molecularweight epoxy resin is soluble in ketone solvents in spite of its highstyrene-conversion average molecular weight of 110,000, but on thecontrary, a linear high molecular weight epoxy resin is insoluble inmethyl ethyl ketone in spite of its lower styrene-conversion weightaverage molecular weight of 66,000. Linear high molecular weight epoxyresins cannot be dissolved completely in methyl ethyl ketone unless theyhave styrene-conversion weight average molecular weights of about 20,000or less.

Among the high molecular weight epoxy resins obtained by theabove-described method, the preferred are linear ultra high molecularweight epoxy resins having styrene-conversion weight average molecularweights of at least 50,000, more preferably at least 100,000, asmeasured by gel permeation chromatography, and reduced viscosities[η_(sp) /c] of at least 0.6 dl/g, as measured in N,N-dimethylacetamideat a concentration of 0.3 g/dl at 30° C.

The high molecular weight epoxy resin to be used in the presentinvention preferably has a viscosity of at least 1,000 mPa.s, morepreferably at least 2,000 mPa.s, as measured in N,N-dimethylacetamide ata concentration of 20% by weight at 25° C.

In the method of the present invention, a solution of the high molecularweight epoxy resin dissolved in an aprotic polar solvent at aconcentration of at most 50% by weight, preferably 15 to 50% by weight,is used to coat the plate having a release surface. A concentration ofmore than 50% by weight may make the viscosity of the solution increaseexcessively and the application of the solution be impossible. Thepreferred examples of the aprotic polar solvent to be used for thepreparation of the solution of the high molecular weight epoxy resin areamide solvents, and the aprotic polar solvents may be identical with ordifferent from the aprotic polar solvent used in the polymerizationreaction.

The solution of the high molecular epoxy resin preferably contains noingredients other than the aprotic polar solvent and the high molecularweight epoxy resin obtained by the polymerization of the difunctionalepoxy resin and the dihydric phenol.

Further, the reaction solution containing the high molecular weightepoxy resin obtained by the polymerization may be used as the solutionof the high molecular weight epoxy resin to be applied to the plate. Inthis case, the reaction solution may be used as it is, or may beconcentrated by removing a portion of the aprotic polar solvent used inthe polymerization or be diluted by adding an aprotic polar solventwhich is identical with or different from the solvent used in thepolymerization in order to adjust the viscosity of the solutionproperly. In this case, the solution preferably contains no ingredientsother than the aprotic polar solvent used in the polymerization, theaprotic polar solvent added, if any, the catalyst used in thepolymerization and the high molecular weight epoxy resin obtained by thepolymerization of the difunctional epoxy resin and the dihydric phenol.

The solution of the high molecular weight epoxy resin is then applied toa plate having a release surface to coat the release surface with thesolution of the high molecular weight epoxy resin.

The plate may be any plate, such as a metal plate, glass plate, ceramicplate or plastics plate, as far as it has a release surface, areresistant to the solvent in the solution and are made of a relativelyhard material. The plate may have a form of a roll.

Subsequently, the aprotic polar solvent is removed from the solution ofthe high molecular weight epoxy resin applied to the plate to form anepoxy resin film with a thickness of not more than 100 μm. The removalof the aprotic polar solvent may be performed by a method of drying byheating or by a method wherein the plate coated with the solution of thehigh molecular weight epoxy resin is dipped in a solvent forsolvent-exchange which is miscible with the aprotic polar solvent butdoes not dissolve the high molecular weight epoxy resin, to performsolvent-exchange, and then drying is carried out by heating.

When the removal of the aprotic polar solvent is performed by drying,the drying temperature is not particularly limited as far as it is nothigher than decomposition temperature of the high molecular weight epoxyresin (generally about 350° C.). The drying may be carried out at areduced pressure. It is preferable to carry out the drying in the air orin an inert atmosphere, for example, in nitrogen gas, argon gas orhelium gas atmosphere.

When the removal of the aprotic polar solvent is performed by the methodemploying the solvent-exchange technique, the solvent to be used for thesolvent-exchange may be any solvent other than aprotic polar solvents asfar as it is miscible with the aprotic polar solvent used in thesolution of the high molecular weight epoxy resin and does not dissolvethe high molecular weight epoxy resin. When the aprotic polar solventused in the solution of the high molecular weight epoxy resin is anamide solvent, some examples of the solvent for solvent-exchange includewater, alcohol solvents such as methanol, ethanol, propanol and butanol,hydrocarbon solvents such as hexane, cyclohexane and toluene, estersolvents such as ethylacetate and propyl acetate, ether solvents such asdiethyl ether and tetrahydrofuran, ketone solvents such as acetone,methyl ethyl ketone, methyl isobutyl ketone and halogenized hydrocarbonsolvents such as chloroform and methylene chloride.

In the step of solvent-exchange, the high molecular weight epoxy resinwhich is coating the surface of the plate in a state of a solutiondissolved in the aprotic polar solvent (good or desired solvent) isdeposited by the excessive amount of solvent for solvent-exchange (bador not desired solvent).

When it is difficult to deposit the high molecular weight epoxy resin bya single solvent exchange using a desired solvent for solvent-exchange,such as a solvent having a low boiling point or water, solvent-exchangemay be carried out once or more by using other solvents which are moremiscible with the aprotic polar solvent than the desired solvent forsolvent-exchange prior to the solvent exchange using the desiredsolvent. For example, when solvent-exchange is carried out twice tocomplete the deposition of the high molecular weight epoxy resin by thesecond solvent-exchange using a desired solvent for solvent-exchange,examples of the combination of two different solvents forsolvent-exchange are an alcohol solvent/water, an alcohol solvent/anether solvent, an alcohol solvent/an ester solvent, and an ethersolvent/a ketone solvent.

When the removal of the aprotic polar solvent is performed by employingthe solvent-exchange and drying, the preferred thickness of the appliedsolution of the high molecular weight epoxy resin is 2 mm or less. Thesolvent-exchange, generally, is carried out for 5 to 120 minutes at 20°to 80° C.

At the time of drying after the solvent-exchange, the drying temperatureis not particularly limited as far as it is not higher than thedecomposition temperature of the high molecular weight epoxy resin(generally about 350° C.). The drying may be carried out at a reducedpressure. It is preferable to carry out the drying in the air or in aninert atmosphere, for example, in nitrogen gas, argon gas or helium gasatmosphere.

Using a solvent for solvent-exchange having a low boiling point enablesdrying at a relatively low temperature. Drying at a relatively lowtemperature makes it extremely easy to endow the dried epoxy resin filmwith adhesion properties. Further, when a solvent with strong polaritysuch as water is used as the solvent for solvent-exchange, metal ionscontained in the raw materials, in particular the catalysts, can bewashed off from the epoxy resin film, and the electric insulatingproperties can be increased.

Because the epoxy resin film of the present invention is produced byusing a substantially linear high molecular weight epoxy resin which issubstantially free from branching, film formation can be performedwithout modifying the high molecular weight epoxy resin with otherresins. The epoxy resin film obtained by the present invention has ahigh glass transition temperature, has a high mechanical strength evenif it is thin, and is excellent in adhesion properties because itcontains many polar groups in its skeleton structure.

That is, according to the method of the present invention, an epoxyresin film of at most 100 μm thickness having a tensile strength of 10MPa or more, further 20 MPa or more, and an elongation of 5% or more,further 10% or more, furthermore 50% or more can be obtained. Further,it is possible to obtain an epoxy resin film having a glass transitiontemperature of 80° C. or more, further 100° C. or more. It is alsopossible to form an epoxy resin film of 50 μm or less thickness.

When a plastic film is used in place of the plate having a releasesurface, a multilayer film having a tensile strength of 10 MPa or more,further 20 MPa or more, and an elongation of 5% or more, further 10% ormore, furthermore 50% or more, can be obtained. The plastic film to becoated with the solution of the high molecular weight epoxy resin is notlimited as far as it is insoluble in methyl ethyl ketone or ethyleneglycol monoethyl ether and the aprotic polar solvent in the solution ofthe high molecular weight epoxy resin. From the view point of thesolvent resistance, heat decomposition temperature and strength, thepreferred examples are polyimide film, polyester film, polyethylene filmand polypropylene film.

In the production of the multilayer film, the drying temperature is notlimited as far as it is lower than the decomposition temperature of thehigh molecular weight epoxy resin and the plastics film.

The epoxy resin film of the present invention is very thin in thicknessand has high strength and high elongation and, therefore, is usefulparticularly as an adhesive film for copper clad laminates and as aninterlaminar material for multilayer boards.

The present invention will be described in more detail with reference tothe following Examples. These Examples, however, are not to be construedto limit the scope of the invention.

EXAMPLES 1 TO 35 AND COMPARATIVE EXAMPLES 1 TO 7

The measurements carried out in the following Examples and ComparativeExamples were conducted in the manner as described below.

The viscosities of the reaction solutions were measured by using anEMD-model viscometer (produced by Tokyo Keiki Co., Ltd.).

Gel permeation chromatography was operated by using a column of TSK gelG6000+G5000+G4000+G3000+G2000. N,N-dimethylacetamide was used as aneluent, with the concentration of samples adjusted to 2% by weight.After the relationship between the molecular weight and the elution timewas obtained by using polystyrenes having various molecular weights, themolecular weights of samples were calculated from their elution time asstyrene-conversion weight average molecular weights.

The photometer used in the determination of the weight average molecularweights by light scattering method was a DLS-700 produced by OhtsukaElectronics Co., Ltd.

The reduced viscosities were measured in N,N-dimethylacetamide as asolvent at a concentration of 0.3 g/dl at 30° C.

Measurements of the tensile strength, elongation and tensile moduluswere conducted by using TENSILON produced by Orientic Co., Ltd. Thesamples of films had a size of 50×10 mm, and the tensile speed was 5mm/min.

The glass transition temperatures (Tg) were measured by using a 910differential scanning calorimeter (DSC) produced by E. I. Du Pont deNemours and Company.

The heat decomposition temperature is the temperature for onset ofreduction in weight in the air which was measured by using adifferential thermobalance, TGD-3000, produced by ULVAC Corp.

EXAMPLES 1 TO 5

Solutions of high molecular weight epoxy resins having differentmolecular weights were prepared by carrying out polymerization under theconditions shown in Table 1 by using, in each Example, 1.000 epoxyequivalent of a bisphenol A epoxy resin (epoxy equivalent weight: 171.5)as a difunctional epoxy resin, bisphenol A (hydroxyl equivalent weight:115.0) as a dihydric phenol in a quantity which provides the ratio ofphenolic hydroxyl groups to epoxy groups shown in Table 1, 1.62 g ofsodium methoxide as a catalyst and N,N-dimethylacetamide as a solvent ina quantity which provides the solids concentration (the proportion ofthe total of bisphenol A epoxy resin and bisphenol A based on the totalof bisphenol A epoxy resin, bisphenol A and N,N-dimethylacetamide) shownin Table 1.

These solutions of high molecular weight epoxy resins were applied toglass plates and then dried by heating in a drier to obtain epoxy resinfilms under the conditions shown in Table 1, and the properties of theobtained epoxy resin films were evaluated. The results are shown inTable 1.

The high molecular weight epoxy resins produced by the abovepolymerization were insoluble in methyl ethyl ketone.

EXAMPLE 6 TO 10

Solutions of high molecular weight epoxy resins having differentmolecular weights were prepared by carrying out polymerization under theconditions shown in Table 2 by using, in each Example, 1.000 epoxyequivalent of a bisphenol A epoxy resin (epoxy equivalent weight: 171.5)as a difunctional epoxy resin, hydroquinone (hydroxyl equivalent weight:55.7) as a dihydric phenol in a quantity which provides the ratio ofphenolic hydroxyl groups to epoxy groups shown in Table 2, 0.72 g oflithium hydroxide as a catalyst and N,N-dimethylacetamide as a solventin a quantity which provides the solids concentration (the proportion ofthe total of bisphenol A epoxy resin and hydroquinone based on the totalof bisphenol A epoxy resin, hydroquinone and N,N-dimethylacetamide)shown in Table 2.

These solutions of high molecular weight epoxy resins were applied toglass plates and then dried by heating in a drier to obtain epoxy resinfilms under the conditions shown in Table 2, and the properties of theobtained epoxy resin films were evaluated. The results are shown inTable 2.

The high molecular weight epoxy resins produced by the abovepolymerization were insoluble in methyl ethyl ketone.

EXAMPLES 11 TO 15

Solutions of high molecular weight epoxy resins having differentmolecular weights were prepared by carrying out polymerization under theconditions shown in Table 3 by using, in each Example, 1.000 epoxyequivalent of a bisphenol A epoxy resin (epoxy equivalent weight: 171.5)as a difunctional epoxy resin, resorcinol (hydroxyl equivalent weight:55.4) as a dihydric phenol in a quantity which provides the ratio ofphenolic hydroxyl groups to epoxy groups shown in Table 3, 1.20 g ofsodium hydroxide as a catalyst and N,N-dimethylacetamide as a solvent ina quantity which provides the solids concentration (the proportion ofthe total of bisphenol A epoxy resin and resorcinol based on the totalof bisphenol A epoxy resin, resorcinol and N,N-dimethylacetamide) shownin Table 3.

These solutions of high molecular weight epoxy resins were applied tostainless steel plates and then dried by heating in a drier to obtainepoxy resin films under the conditions shown in Table 3, and theproperties of the obtained epoxy resin films were evaluated. The resultsare shown in Table 3.

The high molecular weight epoxy resins produced by the abovepolymerization were insoluble in methyl ethyl ketone.

EXAMPLES 16 TO 20

Solutions of high molecular weight epoxy resins having differentmolecular weights were prepared by carrying out polymerization under theconditions shown in Table 4 by using, in each Example, 1.000 epoxyequivalent of a bisphenol A epoxy resin (epoxy equivalent weight: 171.5)as a difunctional epoxy resin, hydroquinone (hydroxyl equivalent weight:55.7) as a dihydric phenol in a quantity which provides the ratio ofphenolic hydroxyl groups to epoxy groups shown in Table 4, 1.62 g ofsodium methoxide as a catalyst and N-methylpyrrolidone as a solvent in aquantity which provides the solids concentration (the proportion of thetotal of bisphenol A epoxy resin and hydroquinone based on the total ofbisphenol A epoxy resin, hydroquinone and N-methylpyrrolidone) shown inTable 4.

These solutions of high molecular weight epoxy resins were applied toglass plates and then dried by heating in a drier to obtain epoxy resinfilms under the conditions shown in Table 4, and the properties of theobtained epoxy resin films were evaluated. The results are shown inTable 4.

The high molecular weight epoxy resins produced by the abovepolymerization were insoluble in methyl ethyl ketone.

EXAMPLES 21 TO 25

Solutions of high molecular weight epoxy resins having differentmolecular weights were prepared by carrying out polymerization under theconditions shown in Table 5 by using, in each Example, 1.000 epoxyequivalent of a bisphenol A epoxy resin (epoxy equivalent weight: 179.2)as a difunctional epoxy resin, bisphenol A (hydroxyl equivalent weight:115.0) as a dihydric phenol in a quantity which provides the ratio ofphenolic hydroxyl groups to epoxy groups shown in Table 5, 1.62 g ofsodium methoxide as a catalyst and N,N-dimethylacetamide as a solvent ina quantity which provides the solids concentration (the proportion ofthe total of bisphenol A epoxy resin and bisphenol A based on the totalof bisphenol A epoxy resin, bisphenol A and N,N-dimethylacetamide) shownin Table 5.

These solutions of high molecular weight epoxy resins were applied toglass plates and then dried by heating in a drier to obtain epoxy resinfilms under the conditions shown in Table 5, and the properties of theobtained epoxy resin films were evaluated. The results are shown inTable 5.

The high molecular weight epoxy resins produced by the abovepolymerization were insoluble in methyl ethyl ketone.

The effects of the present invention taken in these Examples wereconfirmed by the following Comparative Examples.

COMPARATIVE EXAMPLES 1 TO 5

Solutions of high molecular weight epoxy resins having differentmolecular weights were prepared by carrying out polymerization under theconditions shown in Table 6 by using, in each Example, 1.000 epoxyequivalent of a bisphenol A epoxy resin (epoxy equivalent weight: 171.5)as a difunctional epoxy resin, bisphenol A (hydroxyl equivalent weight:115.0) as a dihydric phenol in a quantity which provides the ratio ofphenolic hydroxyl groups to epoxy groups shown in Table 6, 1.62 g ofsodium methoxide as a catalyst and N,N-dimethylacetamide (ComparativeExamples 1-3), methyl isobutyl ketone (Comparative Example 4) orethylene glycol monoethyl ether (Comparative Example 5) as a solvent ina quantity which provides the solids concentration (the proportion ofthe total of bisphenol A epoxy resin and bisphenol A based on the totalof bisphenol A epoxy resin, bisphenol A and N,N-dimethylacetamide) shownin Table 6. The obtained high molecular weight epoxy resins were solublein methyl ethyl ketone.

These solutions of high molecular weight epoxy resins were applied toglass plates and then dried by heating in a drier to obtain epoxy resinfilms under the conditions shown in Table 6, and the properties of theobtained epoxy resin films were evaluated. The results are shown inTable 6. In Comparative Examples 1 to 3, films of 100 μm or less couldnot be formed.

COMPARATIVE EXAMPLE 6

The average molecular weight of a commercial phenoxy resin, YP-50(trademark, produced by Tohto Kasei Co., Ltd.), which is a highmolecular weight epoxy resin, was determined. The styrene-conversionweight average molecular weight of the epoxy resin was 68,000 asdetermined by gel permeation chromatography and 77,000 as determined bylight scattering method. The phenoxy resin had a reduced viscosity of0.488 dl/g. The phenoxy resin was dissolved in methyl ethyl ketoneeasily. A 20% by weight solution of the phenoxy resin inN,N-dimethylacetamide had a viscosity of 200 mPa.s at 25° C. Thesolution of the phenoxy resin was applied to a glass plate and dried byheating in an drier in order to form epoxy resin films, but films havinga thickness of 100 μm or less could not be obtained.

COMPARATIVE EXAMPLE 7

The average molecular weight of a commercial diallyl phthalate resin,DT170 (trademark, produced by Tohto Kasei Co., Ltd.), which is a highmolecular weight epoxy resin, was determined. The styrene-conversionweight average molecular weight of the epoxy resin was 145,000 asdetermined by gel permeation chromatography and 61,000 as determined bylight scattering method. The diallyl phthalate resin had a reducedviscosity of 0.105 dl/g. The diallyl phthalate resin was dissolved inmethyl ethyl ketone easily. A 30% by weight solution of the diallylphthalate resin in N,N-dimethylacetamide had a viscosity of 100 mPa·s.The solution of the diallyl phthalate resin was applied to a glass plateand dried by heating in an drier in order to form epoxy resin films, butfilms having a thickness of 100 μm or less could not be obtained.

                                      TABLE 1                                     __________________________________________________________________________                       Example 1                                                                            Example 2                                                                            Example 3                                                                            Example 4                                                                            Example 5                      __________________________________________________________________________    Polymerization conditions                                                     Ratio of the dihydric phenol                                                                     1.000  1.000  0.998  1.002  1.005                          Solids concentration (% by weight)                                                               20     20     20     20     20                             Reaction temperature (°C.)                                                                120    120    120    120    120                            Reaction time (h)  2.0    2.0    2.0    2.0    2.0                            Properties of the resultant epoxy resin                                       Viscosity (mPa · s)                                                                     9,100  9,100  14,300 6,400  2,900                          Weight average molecular weight                                               GPC                295,000                                                                              295,000                                                                              421,000                                                                              216,000                                                                              170,000                        Light scattering method                                                                          271,000                                                                              271,000                                                                              383,000                                                                              198,000                                                                              155,000                        Reduced viscosity [η.sub.sp /c] (dl/g)                                                       1.240  1.240  1.305  1.195  1.140                          Film forming conditions                                                       Drying temperature (°C.)                                                                  200    250    250    200    200                            Drying time (h)    1.0    1.0    1.0    1.0    1.0                            Properties of epoxy resin film                                                Thickness (μm)  18     17     22     19     15                             Tensile strength (MPa)                                                                           45.9   52.2   63.8   37.1   30.9                           Elongation (%)     81     72     94     68     52                             Tensile modulus (MPa)                                                                            1580   1670   1980   1490   1390                           Glass transition temperature (°C.)                                                        113    117    121    107    105                            Heat decomposition temperature (°C.)                                                      348    345    352    336    333                            __________________________________________________________________________      In Tables 1 to 6, Viscosity means the viscosity of 20% by weight solutio     of the obtained high molecular weight epoxy resin in N,Ndimethylacetamide     measured at 25° C.                                                

                                      TABLE 2                                     __________________________________________________________________________                       Example 6                                                                            Example 7                                                                            Example 8                                                                            Example 9                                                                            Example 10                     __________________________________________________________________________    Polymerization conditions                                                     Ratio of the dihydric phenol                                                                     1.002  1.005  1.010  1.015  1.020                          Solids concentration (% by weight)                                                               20     20     20     20     20                             Reaction temperature (°C.)                                                                120    120    120    120    120                            Reaction time (h)  2.5    2.5    2.5    2.5    2.5                            Properties of the resultant epoxy resin                                       Viscosity (mPa · s)                                                                     14,300 10,900 8,700  5,100  2,400                          Weight average molecular weight                                               GPC                769,000                                                                              625,000                                                                              576,000                                                                              436,000                                                                              307,000                        Light scattering method                                                                          693,000                                                                              559,000                                                                              512,000                                                                              385,000                                                                              255,000                        Reduced viscosity [η.sub.sp /c] (dl/g)                                                       1.325  1.295  1.270  1.240  1.165                          Film forming conditions                                                       Drying temperature (°C.)                                                                  200    200    200    200    200                            Drying time (h)    1.0    1.0    1.0    1.0    1.0                            Properties of epoxy resin film                                                Thickness (μm)  22     24     19     21     17                             Tensile strength (MPa)                                                                           48.1   45.6   40.6   41.2   32.7                           Elongation (%)     165    101    128    76     79                             Tensile modulus (MPa)                                                                            930    1210   750    760    480                            Glass transition temperature (°C.)                                                        88     84     89     81     75                             Heat decomposition temperature (°C.)                                                      326    320    331    329    323                            __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                       Example 11                                                                           Example 12                                                                           Example 13                                                                           Example 14                                                                           Example 15                     __________________________________________________________________________    Polymerization conditions                                                     Ratio of the dihydric phenol                                                                     1.000  1.002  1.005  1.010  1.020                          Solids concentration (% by weight)                                                               20     20     20     20     20                             Reaction temperature (°C.)                                                                120    120    120    120    120                            Reaction time (h)  3.5    3.5    3.5    3.5    3.5                            Properties of the resultant epoxy resin                                       Viscosity (mPa · s)                                                                     2,900  2,200  1,900  1,100  700                            Weight average molecular weight                                               GPC                451,000                                                                              389,000                                                                              326,000                                                                              211,000                                                                              155,000                        Light scattering method                                                                          401,000                                                                              347,000                                                                              292,000                                                                              182,000                                                                              130,000                        Reduced viscosity [η.sub.sp /c] (dl/g)                                                       1.240  1.212  1.200  1.095  1.010                          Film forming conditions                                                       Drying temperature (°C.)                                                                  200    200    200    200    200                            Drying time (h)    1.0    1.0    1.0    1.0    1.0                            Properties of epoxy resin film                                                Thickness (μm)  25     21     20     22     20                             Tensile strength (MPa)                                                                           42.9   46.3   40.1   38.7   29.1                           Elongation (%)     133    156    220    108    83                             Tensile modulus (MPa)                                                                            960    890    1090   1010   780                            Glass transition temperature (°C.)                                                        83     80     82     75     77                             Heat decomposition temperature (°C.)                                                      330    329    329    337    335                            __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________                       Example 16                                                                           Example 17                                                                           Example 18                                                                           Example 19                                                                           Example 20                     __________________________________________________________________________    Polymerization conditions                                                     Ratio of the dihydric phenol                                                                     0.998  1.000  1.002  1.005  1.010                          Solids concentration (% by weight)                                                               20     20     20     20     20                             Reaction temperature (°C.)                                                                120    120    120    120    120                            Reaction time (h)  2.0    2.0    2.0    2.0    2.0                            Properties of the resultant epoxy resin                                       Viscosity (mPa · s)                                                                     8,200  4,000  3,200  2,500  2,000                          Weight average molecular weight                                               GPC                189,000                                                                              215,000                                                                              201,000                                                                              140,000                                                                              117,000                        Light scattering method                                                                          179,000                                                                              202,000                                                                              195,000                                                                              114,000                                                                              110,000                        Reduced viscosity [η.sub.sp /c] (dl/g)                                                       0.942  1.010  1.035  0.910  0.880                          Film forming conditions                                                       Drying temperature (°C.)                                                                  250    250    250    250    250                            Drying time (h)    1.0    1.0    1.0    1.0    1.0                            Properties of epoxy resin film                                                Thickness (μm)  18     13     13     16     12                             Tensile strength (MPa)                                                                           50.4   58.1   50.1   41.9   25.7                           Elongation (%)     79     44     99     56     78                             Tensile modulus (MPa)                                                                            1290   1100   1330   930    500                            Glass transition temperature (°C.)                                                        101    100    107    98     90                             Heat decomposition temperature (°C.)                                                      358    349    350    344    348                            __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________                       Example 21                                                                           Example 22                                                                           Example 23                                                                           Example 24                                                                           Example 25                     __________________________________________________________________________    Polymerization conditions                                                     Ratio of the dihydric phenol                                                                     0.990  0.995  1.000  1.005  1.010                          Solids concentration (% by weight)                                                               20     20     20     20     20                             Reaction temperature (°C.)                                                                120    120    120    120    120                            Reaction time (h)  2.0    2.0    2.0    2.0    2.0                            Properties of the resultant epoxy resin                                       Viscosity (mPa · s)                                                                     3,800  4,800  3,000  1,900  700                            Weight average molecular weight                                               GPC                88,000 119,000                                                                              114,000                                                                              94,000 63,000                         Light scattering method                                                                          89,000 115,000                                                                              109,000                                                                              96,000 60,000                         Reduced viscosity [η.sub.sp /c] (dl/g)                                                       0.805  0.952  0.903  0.854  0.880                          Film forming conditions                                                       Drying temperature (°C.)                                                                  200    250    250    200    200                            Drying time (h)    1.0    1.0    1.0    1.0    1.0                            Properties of epoxy resin film                                                Thickness (μm)  19     23     25     17     16                             Tensile strength (MPa)                                                                           34.9   31.0   25.7   22.6   20.0                           Elongation (%)     66     61     80     60     51                             Tensile modulus (MPa)                                                                            390    400    570    300    260                            Glass transition temperature (°C.)                                                        98     91     94     87     89                             Heat decomposition temperature (°C.)                                                      339    340    341    337    335                            __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________                       Comparative                                                                          Comparative                                                                          Comparative                                                                          Comparative                                                                          Comparative                                       Example 1                                                                            Example 2                                                                            Example 3                                                                            Example 4                                                                            Example 5                      __________________________________________________________________________    Polymerization conditions                                                     Ratio of the dihydric phenol                                                                     0.700  0.800  0.850  1.000  1.000                          Solids concentration (% by weight)                                                               20     20     20     20     20                             Reaction temperature (°C.)                                                                100    100    100    100    100                            Reaction time (h)  1.5    2.5    3.0    2.0    3.5                            Properties of the resultant epoxy resin                                       Viscosity (mPa · s)                                                                     600    900    2,800  900    1,100                          Weight average molecular weight                                               GPC                123,000                                                                              155,000                                                                              111,000                                                                              35,000 67,000                         Light scattering method                                                                          101,000                                                                              138,000                                                                              110,000                                                                              38,000 66,000                         Reduced viscosity [η.sub.sp /c] (dl/g)                                                       0.459  0.550  0.555  0.450  0.509                          Film forming conditions                                                       Drying temperature (°C.)                                                                  200    200    200    200    200                            Drying time (h)    1.0    1.0    1.0    1.0    1.0                            Properties of epoxy resin film                                                Thickness (μm)  --     --     --     27     25                             Tensile strength (MPa)                                                                           --     --     --     6.8    9.0                            Elongation (%)     --     --     --     13     32                             Tensile modulus (MPa)                                                                            --     --     --     150    345                            Glass transition temperature (° C.)                                                       --     --     --     88     95                             Heat decomposition temperature (°C.)                                                      --     --     --     340    338                            __________________________________________________________________________

In Comparative Examples 1 to 3, it is presumed that the excessiveamounts of the epoxy resins caused much branching, and no films of 100μm or less thickness could be obtained although the obtained highmolecular weight epoxy resins had considerably high molecular weight of100,000 or more.

In Comparative Examples 4 and 5 wherein solvents other than aproticpolar solvents were used as polymerization solvents, the obtained highmolecular weight epoxy resins were soluble in methyl ethyl ketone, andthe films formed from the epoxy resins had low tensile strengths.

Also, as described in Comparative Examples 6 and 7, although the phenoxyresin and diallyl phthalate resin, which are commercial bisphenol Aultra high molecular weight epoxy resins, had considerably highmolecular weights, these resins were dissolved in methyl ethyl ketone,and the viscosities of the 20% by weight solution of these resins inN,N-dimethylacetamide were extremely lower than those of the ultramolecular weight epoxy resins obtained according to the presentinvention. These commercial resins also could not be formed into filmshaving a thickness of 100 μm or less.

As another Comparative Example, an epoxy resin having a relatively lowmolecular weight was prepared by using the same reaction materials andin the same quantities as those employed in Example 1. The epoxy resincould be formed into a film, but had an extremely low strength.

On the contrary to Comparative Examples, all Examples could provideepoxy resin films having thicknesses of less than 100 μm and sufficientstrength.

EXAMPLE 26

171.3 g of a bisphenol A epoxy resin (epoxy equivalent weight:171.3) asa difunctional epoxy resin, 115.5 g of bisphenol A (hydroxyl equivalentweight:115.5) as a dihydric phenol and 1.62 g of sodium methoxide as acatalyst were dissolved in N,N-dimethylacetamide as a solvent to preparea reaction solution having a solids concentration (the proportion of thetotal of the bisphenol A epoxy resin and bisphenol A based on the totalof the bisphenol A epoxy resin, bisphenol A and N,N-dimethylacetamide)of 20% by weight. The temperature of the reaction system was maintainedat 120° C. for 6 hours to obtain a solution of a high molecular weightepoxy resin. The obtained high molecular weight epoxy resin had aviscosity of 6,800 mPa.s as measured in N,N-dimethylacetamide at aconcentration of 20% by weight at 25° C., a styrene-conversion weightaverage molecular weight of 295,000 as determined by gel permeationchromatography and a weight average molecular weight of 271,000 asdetermined by light scattering method. Further, the high molecularweight epoxy resin had a reduced viscosity of 1.050 dl/g as measured inN,N-dimethylacetamide at a concentration of 0.3 g/dl at 30° C. The highmolecular weight epoxy resin was insoluble in methyl ethyl ketone.

After the resultant reaction solution containing the high molecularweight epoxy resin was applied to a surface of a glass plate, the glassplate was dipped gently in a large amount of distilled water which was afirst solvent for solvent-exchange. The glass plate coated with thesolution of the high molecular weight epoxy resin was taken out 30minutes after, and was then immediately dipped gently in a large amountof a hot distilled water (90° C.) which was a second solvent forsolvent-exchange. The glass plate was taken out 30 minutes after, and,subsequently, an epoxy resin film formed on the glass plate was peeledoff from the glass plate and was then dried by heating at 105° C. forone hour in a drier.

The epoxy resin film was white and opaque immediately after it had beentaken out from the hot distilled water, but became light yellow andtransparent after the drying by heating. The resultant epoxy resin filmhad a thickness of 28 μm, a tensile strength of 18 MPa and an elongationof 260%, and could attach two stainless steel plates together by usingpressure at 180° C.

EXAMPLE 27

The procedure of Example 26 was repeated with the exception thatmethanol was used as a first solvent for solvent-exchange in place ofthe distilled water, hexane was used as a second solvent forsolvent-exchange in place of the hot distilled water, and the dryingtemperature was changed to 80° C., to obtain an epoxy resin film.

The resultant epoxy resin film had a thickness of 25 μm, a tensilestrength of 25 MPa and an elongation of 180%, and could attach twostainless steel plates together by using pressure at 150° C.

EXAMPLE 28

The procedure of Example 26 was repeated with the exception thattetrahydrofuran was used as a first solvent for solvent-exchange inplace of the distilled water, diethyl ether was used as a second solventfor solvent-exchange in place of the hot distilled water, and the dryingtemperature was changed to 70° C., to obtain an epoxy resin film.

The resultant epoxy resin film had a thickness of 21 μm, a tensilestrength of 17 MPa and an elongation of 190%, and could attach twostainless steel plates together by using pressure at 130° C.

EXAMPLE 29

173.2 g of a bisphenol A epoxy resin (epoxy equivalent weight: 173.2) asa difunctional epoxy resin, 55.2 g of hydroquinone (hydroxyl equivalentweight: 55.2) as a dihydric phenol and 0.72 g of lithium hydroxide as acatalyst were dissolved in N,N-dimethylacetamide as a solvent to preparea reaction solution having a solids concentration (the proportion of thetotal of the bisphenol A epoxy resin and hydroquinone based on the totalof the bisphenol A epoxy resin, hydroquinone and N,N-dimethylacetamide)of 20% by weight. The temperature of the reaction system was maintainedat 110° C. for 6 hours to obtain a solution of a high molecular weightepoxy resin. The obtained high molecular weight epoxy resin had aviscosity of 10,900 mPa.s as measured in N,N-dimethylacetamide at aconcentration of 20% by weight at 25° C., a styrene-conversion weightaverage molecular weight of 625,000 as determined by gel permeationchromatography and a weight average molecular weight of 559,000 asdetermined by light scattering method. Further, the high molecularweight epoxy resin had a reduced viscosity of 1.240 dl/g as measured inN,N-dimethylacetamide at a concentration of 0.3 g/dl at 30° C. The highmolecular weight epoxy resin was insoluble in methyl ethyl ketone.

After the resultant reaction solution containing the high molecularweight epoxy resin was applied to a surface of a glass plate, the glassplate coated with the solution was dipped gently in a large amount ofmethanol which was a first solvent for solvent-exchange. The glass platewas taken out 30 minutes after, and was then immediately dipped gentlyin a large amount of hot distilled water (90° C.) which was a secondsolvent for solvent-exchange. The glass plate was taken out 30 minutesafter, and, subsequently, an epoxy resin film formed on the glass platewas peeled off from the glass plate and was then dried by heating at105° C. for one hour in a drier.

The resultant epoxy resin film had a thickness of 31 μm, a tensilestrength of 22 MPa and an elongation of 420%, and could attach twostainless steel plates together by using pressure at 180° C.

EXAMPLE 30

173.2 g of a bisphenol A epoxy resin (epoxy equivalent weight: 173.2) asa difunctional epoxy resin, 55.2 g of resorcinol (hydroxyl equivalentweight: 55.2) as a dihydric phenol and 1.2 g of sodium hydroxide as acatalyst were dissolved in N,N-dimethylacetamide as a solvent to preparea reaction solution having a solids concentration (the proportion thetotal of the bisphenol A epoxy resin and resorcinol based on the totalof the bisphenol A epoxy resin, resorcinol and N,N-dimethylacetamide) of20% by weight. The temperature of the reaction system was maintained at100° C. for 8 hours to obtain a solution of a high molecular weightepoxy resin. The obtained high molecular weight epoxy resin had aviscosity of 2,900 mPa.s as measured in N,N-dimethylacetamide at aconcentration of 20% by weight at 25° C., a styrene-conversion weightaverage molecular weight of 451,000 as determined by gel permeationchromatography and a weight average molecular weight of 401,000 asdetermined by light scattering method. Further, the high molecularweight epoxy resin had a reduced viscosity of 1.185 dl/g as measured inN,N-dimethylacetamide at a concentration of 0.3 g/dl at 30° C. The highmolecular weight epoxy resin was insoluble in methyl ethyl ketone.

After the resultant reaction solution containing the high molecularweight epoxy resin was applied to a surface of a glass plate, the glassplate coated with the solution was dipped gently in a large amount ofacetone which was a first solvent for solvent-exchange. The glass platewas taken out 30 minutes after, and was then immediately dipped gentlyin a large amount of chloroform which was a second solvent forsolvent-exchange. The glass plate was taken out 30 minutes after, and,subsequently, an epoxy resin film formed on the glass plate was peeledoff from the glass plate and was then dried by heating at 80° C. for onehour in a drier.

The resultant epoxy resin film had a thickness of 22 μm, a tensilestrength of 27 MPa and an elongation of 150%, and could attach twostainless steel plates together by using pressure at 150° C.

EXAMPLE 31

171.3 g of a bisphenol A epoxy resin (epoxy equivalent weight: 171.3) asa difunctional epoxy resin, 115.5 g of bisphenol A (hydroxyl equivalentweight: 115.5) as a dihydric phenol and 1.62 g of sodium methoxide as acatalyst were dissolved in N,N-dimethylacetamide as a solvent to preparea reaction solution having a solids concentration (the proportion of thetotal of the bisphenol A epoxy resin and bisphenol A based on the totalof the bisphenol A epoxy resin, bisphenol A and N,N-dimethylacetamide)of 20% by weight. The temperature of the reaction system was maintainedat 120° C. for 6 hours to obtain a 20% by weight solution of a highmolecular weight epoxy resin. The obtained high molecular weight epoxyresin had a viscosity of 6,800 mPa·s as measured inN,N-dimethylacetamide at a concentration of 20% at 25° C. and wasinsoluble in methyl ethyl ketone.

The obtained solution of the high molecular weight epoxy resin wasapplied to both surfaces of a polyimide film of 25 μm thickness, and wasthen dried by heating at 200° C. for one hour in a drier to obtain anepoxy resin multilayer film of 39 μm thickness, and the properties ofthe multilayer film were evaluated. The multilayer film was transparentand good in appearance and had a tensile strength of 85.2 MPa, anelongation of 39.0%, a tensile modulus of 950 MPa and a heatdecomposition temperature of 355° C.

EXAMPLE 32

173.2 g of a bisphenol A epoxy resin (epoxy equivalent weight: 173.2) asa difunctional epoxy resin, 55.2 g of resorcinol (hydroxyl equivalentweight: 55.2) as a dihydric phenol and 1.20 g of sodium hydroxide as acatalyst were dissolved in N,N-dimethylacetamide as a solvent to preparea reaction solution having a solids concentration (the proportion of thetotal of the bisphenol A epoxy resin and resorcinol based on the totalof the bisphenol A epoxy resin, resorcinol and N,N-dimethylacetamide) of20% by weight. The temperature of the reaction system was maintained at100° C. for 8 hours to obtain a 20% by weight solution of a highmolecular weight epoxy resin. The obtained high molecular weight epoxyresin had a viscosity of 2,200 mPa·s as measured inN,N-dimethylacetamide at a concentration of 20% at 25° C. and wasinsoluble in methyl ethyl ketone.

The obtained solution of the high molecular weight epoxy resin wasapplied to both surfaces of a polyester film of 25 μm thickness, and wasthen dried by heating at 200° C. for one hour in a drier to obtain anepoxy resin multilayer film of 45 μm thickness, and the properties ofthe multilayer film were evaluated. The multilayer film was transparentand good in appearance and had a tensile strength of 64.7 MPa, anelongation of 48.5%, a tensile modulus of 1130 MPa and a heatdecomposition temperature of 345° C.

EXAMPLE 33

171.3 g of a bisphenol A epoxy resin (epoxy equivalent weight: 171.3) asa difunctional epoxy resin, 272.0 g of tetrabromobisphenol A (hydroxyequivalent weight: 272.0) as a dihydric phenol and 0.72 g of lithiumhydroxide as a catalyst were dissolved in N,N-dimethylacetamide as asolvent to prepare a reaction solution having a solids concentration(the proportion of the total of the bisphenol A epoxy resin andtetrabromobisphenol A based on the total of the bisphenol A epoxy resin,tetrabromobisphenol A and N,N-dimethylacetamide) of 30% by weight. Thetemperature of the reaction system was maintained at 120° C. for 6 hoursto obtain a solution of a high molecular weight epoxy resin. Theobtained high molecular weight epoxy resin had a viscosity of 9,600mPa·s as measured in N,N-dimethylacetamide at a concentration of 30% byweight at 25° C., a styrene-conversion weight average molecular weightof 195,000 as determined by gel permeation chromatography and a weightaverage molecular weight of 221,000 as determined by light scatteringmethod. Further, the high molecular weight epoxy resin had a reducedviscosity of 1,000 dl/g as measured in N,N-dimethylacetamide at aconcentration of 0.3 g/dl at 30° C. The high molecular weight epoxyresin was insoluble in methyl ethyl ketone.

After the resultant reaction solution containing the high molecularweight epoxy resin was applied to a surface of a glass plate, the glassplate was dipped gently in a large amount of distilled water which was afirst solvent for solvent-exchange. The glass plate coated with thesolution of the high molecular weight epoxy resin was taken out 30minutes after, and was then immediately dipped gently in a large amountof a hot distilled water (90° C.) which was a second solvent forsolvent-exchange. The glass plate was taken out 30 minutes after, and,subsequently, an epoxy resin film formed on the glass plate was peeledoff from the glass plate and was then dried by heating at 105° C. forone hour in a drier.

The epoxy resin film was white and opaque immediately after it had beentaken out from the hot distilled water, but became light yellow andtransparent after the drying by heating. The resultant epoxy resin filmhad a thickness of 28μm, a tensile strength of 28 MPa and an elongationof 8.0%, and could attach two stainless steel plates together by usingpressure at 180° C.

EXAMPLE 34

The procedure of Example 33 was repeated with the exception thatmethanol was used as a first solvent for solvent-exchange in place ofthe distilled water, hexane was used as a second solvent forsolvent-exchange in place of the hot distilled water, and the dryingtemperature was changed to 80° C., to obtain an epoxy resin film.

The resultant epoxy resin film had a thickness of 25 μm, a tensilestrength of 35 MPa and an elongation of 10%, and could attach twostainless steel plates together by using pressure at 150° C.

EXAMPLE 35

The procedure of Example 33 was repeated with the exception thattetrahydrofuran was used as a first solvent for solvent-exchange inplace of the distilled water, diethyl ether was used as a second solventfor solvent-exchange in place of the hot distilled water, and the dryingtemperature was changed to 70° C., to obtain an epoxy resin film.

The resultant epoxy resin film had a thickness of 21 μm, a tensilestrength of 27 MPa and an elongation of 9.0%, and could attach twostainless steel plates together by using pressure at 130° C.

What is claimed is:
 1. A method of producing an epoxy resin film whichis made of a high molecular weight epoxy resin, the high molecularweight epoxy resin being insoluble in methyl ethyl ketone and having asubstantially linear structure, and the epoxy resin film having atensile strength of not less than 10 MPa and an elongation of not lessthan 10%, comprisingapplying a solution of the high molecular weightepoxy resin, having a reduced viscosity (η sp/c) of at least 0.6 dl/g asmeasured in N,N-dimethylacetamide at a concentration of 0.3 g/dl at 30°C., dissolved in an aprotic polar solvent at a concentration of at most50% by weight to a plate having a release surface to coat the releasesurface with the solution of the high molecular weight epoxy resin, thehigh molecular weight epoxy resin being prepared by polymerizing adifunctional epoxy resin having two epoxy groups per molecule and adihydric phenol in an aprotic polar solvent, at a reaction temperatureranging from 80 to 130° C., in the presence of a catalyst selected fromthe group consisting of an alkali metal compound, an alkyl phosphine andan aliphatic cyclic amine, the difunctional epoxy resin and the dihydricphenol being used in quantities which provide a ratio of phenolichydroxyl groups to epoxy groups of from 1:0.9 to 1:1.1 and a proportionof the total of the difunctional epoxy resin and the dihydric phenol ofat most 50% by weight based on the total of the difunctional epoxyresin, the dihydric phenol and the aprotic polar solvent and removingthe aprotic polar solvent from the solution of the high molecular weightepoxy resin applied to the release surface of the plate, to form epoxyresin film having a thickness of not more than 100 μm; the aprotic polarsolvent being an amide solvent selected from the group consisting ofN-methylformamide, N,N-dimethylformamide, acetamide, N-methylacetamide,N,N-dimethylacetamide, N,N,N',N'-tetramethylurea, 2-pyrrolidone,N-methylpyrrolidone and a carbamate.
 2. The method of claim 1, whereinthe solution of the high molecular weight epoxy resin dissolved in anaprotic polar solvent is a reaction solution resulting from thepolymerization.
 3. The method of claim 1, wherein the removal of theaprotic polar solvent from the solution of the high molecular weightepoxy resin applied to the release surface of the plate is carried outby dipping the plate coated with the solution of the high molecularweight epoxy resin in a solvent for solvent-exchange which is misciblewith the aprotic polar solvent but does not dissolve the high molecularweight epoxy resin, to perform solvent-exchange, and then drying iscarried out by heating, to form the epoxy resin film having a thicknessof not more than 100 μm.
 4. The method of claim 3, wherein the solventfor solvent-exchange is a solvent having a boiling point lower than theboiling point of the aprotic polar solvent used in the polymerization.5. The method of claim 4, wherein the solvent for solvent-exchange isselected from the group consisting of water, methanol, hexane,tetrahydrofuran, diethyl ether, acetone, chloroform and ethylacetate. 6.The method of claim 1, wherein the catalyst is an alkali metal compoundselected from the group consisting of hydroxide, halide, an organic acidsalt, an alcoholate, a phenolate, hydride, borohydride and an amide ofsodium, lithium or potassium.
 7. The method of claim 1, wherein thedifunctional epoxy resin is selected from the group consisting ofbisphenol A epoxy resin, bisphenol F epoxy resin, bisphenol S epoxyresin, an alicyclic epoxy resin, an aliphatic linear epoxy resin, adiglycidyl ether of a dihydric phenol, a diglycidyl ether of a dihydricalcohol, a halide thereof, and a hydride thereof.
 8. The method of claim1, wherein the dihydric phenol is selected from the group consisting ofhydroquinone, resorcinol, catechol, bisphenol A, bisphenol F, a halidethereof and an alkyl substituted compound thereof.
 9. The method ofclaim 1, wherein the difunctional epoxy resin is bisphenol A epoxyresin, the dihydric alcohol is hydroquinone, resorcinol, bisphenol A ortetrabromobisphenol A, the catalyst is sodium hydroxide, lithiumhydroxide or sodium methoxide, and the aprotic polar solvent isN,N-dimethylacetamide or N-methylpyrrolidone.